6th Year Geography Mr. Mc Mahon
Optional Unit 7: Geoecology: Soils.
Soil Forming Processes & Human Interference in Soil
Characteristics
Unit 7:2
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Statement : Soil Characteristics are affected by their immediate environment
and by a combination of processes operating in that environment, including
human interference.
In this section you will study
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Soil Forming Processes: weathering, soil erosion, leaching, podzolisation,
calcification and lateralistation.
Brown Earth Soils: Case Study
Human interference with soil characteristics: Case Study The Sahel .
Soil Forming Processes
Having looked at the composition and characteristics of soil in unit 7:1 we will now look at soil
forming processes . The processes involved in the creation of soils are
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Weathering
Erosion
Humification
Leaching
Podzolisation
Salinisation
Calcification
Weathering
Weathering plays an important role in the creation of soils. The type of weathering and parent
material determine can determine soil depth, soil type, moisture retention and fertility.
Mechanical Weathering: Freeze Thaw Action and Exfoliation (onion peeling) do not alter the
mineral content of the parent material, they merely break it up. Mechanical weathering creates
shallow soils from loose rocks exposed at the surface.
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Freeze Thaw Action: Takes place in cold climates and at high altitudes creating scree.
This breaks down to provide minerals for the soil.
Exfoliation/Onion Peeling: Rocks contract and expand as temperatures change from hot
to cold. This results in flakes of rocks creating scree. This takes place in tropical climate
and desert areas.
Chemical Weathering: Acid rain does alter the mineral content of parent material. Rock
minerals decompose under carbonation, hydrolysis and oxidation.
How long it takes for rocks to weather depends on their formation. Granite for example takes
longer to degrade than Limestone or Shale.
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Carbonation: Dissolves calcium in limestone and iron in sandstone.
Hydrolosis: Granite breaks down when its feldspar (crystals) asorb water and turn into
Kaolin Clay.
Oxidation: Occurs when rocks are exposed to oxygen in the air, dissolving their iron
content and giving them a reddish tint
Erosion
The formation of soil is influenced by erosion.
Alluvial Soils: Found in the middle and lower courses of rivers. Alluvial soils contain silt particles
1-2mm that come from rock particles that were dissolved by the river in its upper course.
Alluvial soils are rich in nutrients and ideal for agriculture.
Glaciers: Erode soil in one place and deposit it in another. As glaciers and ice sheets moved
they dragged, pushed and pulled ridges of deposition called moraines. As the ice sheets melted
they deposited their material
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Glacial Till/Boulder clay can contain many different types of rock thus determining
mineral content. Glacial till contains a lot of clay soil with a block like structure. This can
be seen in soil profiles along the east coast of Ireland. E.g Killiney Beach.
Fluvo-Glacial Deposits: Deposits of finer glacial material with small particles were
carried away and deposited by glacial melt water giving rise to outwash plains and
eskers. The soils to be found here have a sandy consistency. E.g Curragh Co. Kildare
Wind Erosion: Aeolian Action: Wind erodes topsoil’s that have been loosened due to exposure
created by a lack of vegetation. Wind action is most effective in areas that lack precipitation.
Loess (German meaning Loose) are soils blown by Aeolian action that accumulate in desert
areas e.g. Dust Bowl, USA and central China. Some Loess is glacial in origin having been blown
from outwash plains to gather someplace else e.g. Paris Basin. Well managed Loess can
produce fertile farmland by means of planting vegetation to bind the soil and provide nutrition
and artificial irrigation. Loess soils can be found the the central valley of California.
Humification
Humification is the process by which vegetation is decomposed to produce humus. Humus is a
dark brown/ black material with a jelly like consistency. Humification takes place under broad
leaved vegetation that loses its leaves in winter. Decidious trees (oak, elm chestnut etc..)
provide leaves in abundance. These leaves are called plant litter. A combination of air,
moisture, heat and micro-organisms break the material down to create a fertile top layer of
soil.
Humification is rapid in hot/humid climates where bacteria can thrive and slower in cold
climates . Humus feeds the soil by providing it with nutrients and nitrogen. It also provides food
for the survival of micro-organisms.
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Humus can hold 80/90 % of its weight in water helping to prevent drought.
Dark Humus can warm up cold soils during spring encouraging growth.
Humus can moderate/ neutrailse excessive amounts of acid in soils making them
alkaline.
The Brown earths
of Ireland and
Northern Europe
are rich in humus.
Leaching
Leaching is the process by which rainfall washes minerals deep into soils beyond the reach of
the plant roots creating a hardpan. A hardpan is a layer of solidified minerals that is
impermeable and can lead to waterlogging. Leaching can be an environmental concern as it can
wash agricultural pesticides into the water table and contaminate it.
Leaching occurs where precipitation is greater than evaporation, on steep slopes and uplands.
Podzolisation: Excessive leaching creates podzols. If the soil is a peaty type the rainwater
combines with its acids dissolving any minerals present. This creates podzols which are grey
infertile soils that contain hardpans. Podzols can be found in upland areas and cold climates e.g
parts of Canada and Russia. Areas that contain podzols can be described as marginal being
infertile for agriculture . However, conifers (spruce, fir ) trees can grow in them. Such trees with
needle like leaves leave little nutrition by way of litter .
Lateralisation: This is the extreme leaching that takes place in hot climates. Rainwater washes
the nutrients deep into the soils that lack vegetation. These soils are therefore low in acid. The
lack of acid means the the soils remain full of iron and magnesium with nothing to corrode
them, These minerals give the soil a reddish tint. On exposure to sunlight and high
temperatures the iron can form a hardpan close to the surface.